Acrylonitrile-butadiene-styrene copolymers (hereinafter “ABS resins”) can be produced by grafting an unsaturated nitrile compound (i.e., an acrylonitrile monomer) and an aromatic vinyl compound (i.e., a styrene monomer) in the presence of a butadiene-based rubber polymer. Generally, desired properties of ABS resins can be achieved by controlling the composition of a rubber polymer, a g-ABS resin or a SAN resin used as a matrix resin therein. The ABS resin prepared therefrom can have good physical properties such as processability, impact resistance, rigidity, and especially excellent coloration property and glossy appearance. Accordingly, ABS resins have been widely used in the production of electric or electronic goods and miscellaneous goods. Increasingly, ABS resins are used in multiple applications, and there are accordingly increasing requirements for multi-functional ABS resins (as compared to mono-functional ABS resins).
Generally, some type of connecting part between an external steel sheet and an internal resin sheet in a refrigerator is manufactured by assembling a table board, a lid and a sash. An insulation layer for a refrigerator can be formed by injecting polyurethane forming liquid in the space between an internal box and an external box of the assembled refrigerator and allowing the liquid to foam and solidify in place. The primary components of the polyurethane foam-insulation layer can include a mixture of a polyol and a diisocyanate compound and a foaming agent, such as freon.
However, compounds used in the polyurethane foam-insulation layer may cause chemical erosion at a portion of the internal box of a refrigerator, where stress is concentrated, by contacting the surface of resin during the manufacturing process or while in use, which can result in stress cracking. Therefore, the resin composition used in the manufacture a refrigerator should have stress cracking resistance against these chemical compounds.
ABS resins have been typically used in the manufacture of an internal box of a refrigerator, because ABS resins can have a good balance of physical properties such as rigidity, impact resistance, processability, glossy appearance, and especially excellent chemical resistance against a freon such as CFC-11 which is used as a foaming agent for a rigid polyurethane foam. CFC-11 is being replaced with HCFC-141b at the present time, because of environmental concerns associated with the impact of CFC-11 on the ozone layer in the stratosphere. However, the use of HCFC-141b can also be problematic as a result of possible stress cracks that can appear on the internal box of a refrigerator by melting the resin component.
In order to solve the above problem, Japanese Patent Application Laid-Open No. 2-284906 is directed to an internal box of a refrigerator formed with an ABS resin containing a high amount of a vinyl cyanide compound, and Japanese Patent Application Laid-Open No. 6-262713 is directed to a method of mixing acrylic rubber during the compounding process. However, the resins produced therefrom are susceptible to stress cracking.